1. Field of the Invention
The present invention relates to directional couplers and more specifically to a directional coupler for use in a radio communication apparatus that performs communications using a high-frequency signal.
2. Description of the Related Art
A directional coupler described in Japanese Unexamined Patent Application Publication No. 2013-5076 is available as a related art direction coupler. The directional coupler includes a main line and a sub-line, opposed to each other with an insulation layer interposed therebetween. In this way, the main line and the sub-line are electromagnetically coupled with each other while being also capacitively coupled with each other.
A disadvantage with the directional coupler described in Japanese Unexamined Patent Application Publication No. 2013-5076 is an insufficient directivity. The flow of a signal in an electromagnetic coupled state and a capacitively coupled state is described below. FIG. 16 through FIG. 18 illustrate the flow of the signals in the directional coupler.
An even mode is created in the electromagnetic coupled state, and an odd mode is created in the capacitively coupled state. As illustrated in FIG. 16, in the even mode, electromagnetic induction in the electromagnetic coupled state causes a signal Sig 2 to flow along the sub-line in the direction opposite to the direction of a signal Sig 1 flowing along the main line. As illustrated in FIG. 17 on the other hand, in the odd mode, an electric field caused generated by the capacitive coupling causes a signal Sig 3 to flow in the opposite direction to the direction of the signal Sig 1 along the sub-line and a signal Sig 4 to flow in the same direction as the direction of the signal Sig 1 along the sub-line. As described above, the main line and the sub-line are electromagnetically coupled while also being capacitively coupled. As a result, part of the signal Sig 2 cancels out the signal Sig 4 as illustrated in FIG. 18. A signal Sig 5 generated when the part of the signal Sig 2 cancels out the signal Sig 4 flows along the sub-line in the opposite direction to the direction of the signal Sig 1. The directional coupler is based on the assumption that no signal is output at a terminal of the sub-line to which the signal Sig 4 flows and that a signal is output at a terminal of the sub-line to which the signals Sig 3 and Sig 5 flow. The characteristics that the sub-line of the directional coupler outputs a signal at only one of the two terminals thereof is referred to as directivity of the directional coupler. The directivity may be adjusted by adjusting the degree of electromagnetic coupling and capacitive coupling.
The directional coupler disclosed in Japanese Unexamined Patent Application Publication No. 2013-5076 includes the main line and the sub-line with the planes thereof opposed to each other and has a high degree of capacitive coupling. As a result, the odd mode appears stronger than the even mode in the directional coupler. Since the signals Sig 3 and Sig 4 flow in opposite directions in the odd mode, a desired directivity is difficult to achieve if the odd mode appears stronger than the even mode. The directional coupler disclosed in Japanese Unexamined Patent Application Publication No. 2013-5076 thus suffers from an insufficient directivity.